memory: tegra: Make terga20_mc_reset_ops static

Fix sparse warning:

drivers/memory/tegra/tegra20.c:277:33: warning:
 symbol 'terga20_mc_reset_ops' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>